Musical wind instruments in the “woodwind family” operate on the principle that air is blown over a reed (or a sharp-edged surface, as on a flute) to set into vibration the air column inside an instrument, Along the length of the instrument's body are numerous large holes (hereafter referred to as “tone holes”) that, when closed or covered, increase the length of the air column inside the instrument. The pitch or note that the instrument sounds when played is determined by the length of the air column, and therefore, by the number of tone holes that are closed. Generally, the greater the number of tone holes covered, the lower the pitch that is produced.
Some woodwind instruments (such as the saxophone, clarinet, bassoon, and oboe) have very small holes (typically of less than 0.5 cm in diameter) positioned at special points along the length of the instrument's body, and that function as “octave vents” or “register vents.” These vents work by allowing a very small amount of air pressure to escape from inside the instrument. This localized release partially interrupts the vibration of the air column moving inside the instrument when a musician is playing it. Allowing a small amount of air pressure to escape through the vent causes the frequency of the note being played on the instrument to jump an octave or more higher as a function of the harmonic properties of the air column, which are influenced by a number of factors including the length of the air column and the position of the vent along the air column.
Octave/register vents are normally covered or closed by some device, such as a lever that is pressed by the musician's finger. The lever (often referred to as a “register key”) normally has a flat, resilient or flexible surface at one end (usually a round disk of cork, rubber or leather) that covers the vent, preventing air from escaping out of the vent. The resilient or flexible material (hereafter referred to as a “pad”) covers the vent hole and creates a seal through which air cannot pass. In most cases, an octave/register vent remains closed by the register key and its pad, with a spring holding the key in closed position, until the musician chooses to press the key, thereby opening the vent.